The subject invention is directed toward the art of metal forming and, more particularly, to an improved foil forming method and apparatus.
The invention is especially suited for forming very thin metal strips or foils and will be described with particular reference thereto; however, as will be appreciated, the invention is capable of broader application and could be used for forming strips or sheets of a range of thicknesses.
As used in the subject specification and claims, the term "strip" is used to identify the thicker input material to the rolling process and the term "foil" is used to identify the thinner output or product. Typically, the term foil is limited to material of less than 6 mils in thickness. And, although the invention is particularly suited for forming metals to a thickness of less than 6 mils, as used in the subject specification and claims foil is also intended to encompass an output or product which can have a thickness somewhat greater than 6 mils.
In the past, thin metal strip or foil has been formed by passing a single metal strip through a series of roll gaps of decreasing spacing. As is well known, the diameter of the rolls forming the roll gap must be related to strip thickness in order for a reduction in thickness to be effected during rolling. That is, as the strip thickness is reduced, the diameters of the rolls forming the roll gap must be reduced.
Within limits, the noted approach works satisfactorily. However, as the thickness decreases, problems are encountered in maintaining a uniform thickness across the width of the strip. This is primarily because the small diameter rolls required for the final reductions lack mechanical stability (e.g. beam strength) and deflect along their length under load. Various types of backup rolls, complicated roll deflecting systems and the like have been used in an attempt to overcome the problem.
The productivity of a rolling mill is inversely related to the strip thickness and the precision and complexity of the rolling mill required increases as the strip thickness decreases. As the strip approaches foil gauges during rolling, the decreased productivity and the increased complexity of the rolling mill become increasingly important factors in the cost of the strip.
An additional approach proposed to permit more economical rolling of thin strip was to roll a plurality of strips simultaneously one upon the other. In this approach, however, mechanical restraints were required to keep the strips aligned during rolling. For example, the strips were enclosed in a casing or some mechanical alignment feature was added to the mill. Also, it was generally necessary to provide some means for preventing the strips from welding together during the rolling. Often, layers of inert material were interposed between the strips or, adherent oxide layers were formed on the surfaces such as in alloys containing chromium.
In certain instances, the above discussed plural strip rolling was more economical than single strip rolling. However, much of the time, the procedures or devices required to maintain alignment and prevent welding were such as to eliminate any possible economic advantages.